Heretofore, it has been attempted to flare an end of a pipe, e.g., a beam member or a frame member in the form of a metallic pipe as a vehicular indoor component or the like, for increasing the mounting strength of the pipe. According to one known machining process for flaring a pipe end, the outer circumferential surface of a blank pipe is clamped in place by clamp dies and a tapered flaring punch is pressed into an end of the pipe hole thereby to flare the pipe end.
When the flaring punch is pressed into the pipe end to flare the same according to the above machining process, the flared pipe end is elongated with a resultant reduction in its thickness. When the elongation limit of the pipe material is reached, the pipe starts being ruptured from the flared end. Ordinary steel pipes can be flared up to diameters that are about 1.4 times those of blank pipes, and cannot further be machined beyond that limit.
When the flaring punch is pressed into an end of a blank pipe, the centers of the blank pipe and the flaring punch may be positioned out of alignment with each other for thereby producing a pipe with an eccentric flared end. According to this process, the produced pipe includes a region which is more elongated than a pipe with a concentric flared end depending on the amount of eccentricity, resulting in large thickness variations of the pipe. Generally, it is more difficult to produce pipes with eccentric flared ends than pipes with concentric flared ends.
FIG. 11 shows measured thicknesses of a pipe with an eccentric flared end which has been machined from a blank pipe having a wall thickness of 1 mm according to the conventional process. The measured thicknesses indicate that the thickness of a pipe region where the wall thickness is reduced to a maximum, among regions in circumferential and axial directions, is reduced to about 50% of the wall thickness of the blank pipe.
Pipes with concentric flared ends and pipes with eccentric flared ends are employed in different spots in vehicles, and some flared ends are attached to vehicle bodies and parts are mounted on other flared ends. Since pipes with ends flared to diameters that are about 1.4 times those of blank pipes suffer insufficient mechanical strength and generally have reduced thickness, they need to be reinforced by separate stiffeners. However, if those pipes are incorporated in vehicles, then use of the stiffeners is disadvantageous because they increase the weight of the vehicles.